Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 161945

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
245 PM CDT Sat Sep 16 2017

244 PM CDT

Through Sunday...

Early afternoon satellite imagery shows the area on the western
flank of an upper ridge with a large upper trough rotating across
the Rockies and High Plains. At the surface, low pressure over
western Lake Superior has a cold front stretching across western
Iowa into Kansas. South to southwest winds are in place across the
local area with a mix of diurnal cumulus and sun overhead. The
rest of the afternoon and evening will remain dry with rather mild
temperatures. Winds will ease somewhat this evening as clouds

Showers and thunderstorms will continue to develop across central
and eastern Iowa and make their way eastward into tonight. The
main upper trough will begin to lift northeast taking the surface
low with it which will stretch the front and weaken its
convergence with time through Sunday. The evening convection will
likely weaken as it moves into northern Illinois but expect that
some showers will linger into the Rockford/I-39 corridor late
tonight with some thunder possible if better organization can
continue. Some of this activity could linger through mid/late
morning into the Chicago area but prospects are not great for this
to occur. The next question becomes how much development occurs
along/ahead of the front Sunday? Low end instability is expected
ahead of the front roughly from Chicago to Pontiac and points east
by early afternoon. Residual cloud cover may limit development
through early afternoon. Chances improve through the afternoon to
the east and it may end up being that any new development is
across northwest Indiana back into Ford/Iroquois Counties. For now
favor some development closer to the Chicago-Pontiac I-55 corridor
with increased coverage further east. Severe potential is on the
low end and heavy downpours and lightning would be the main
threats. Winds shift northwesterly behind the front from northwest
to southeast into early evening.



333 AM CDT

Sunday through Friday...

A cold front moves through the region Sunday.  Isolated showers and
dissipating thunderstorms may bring light rain to areas west of I-55
Sunday morning. Guidance disagrees on how strong forcing will be and
how much convection will form along and east of I-55 Sunday
afternoon. Have medium confidence in scattered or higher storm
coverage Sunday afternoon. The recent dry spell could limit how many
storms form. CAPE values are forecast to stay below 1000 J/kg and
shear less than 20 kt. PWATs on the other hand will be well above
normal around 1.8 inches with only 20 kt steering flow. Therefore,
whatever storms do form may produce heavy rain and move slowly.

The theme of next week will be transient shortwave troughs that
bring multiple chances of showers and storms. Monday evening looks
particularly interesting with a stronger vorticity lobe and short
wave trough reaching the mid Mississippi Valley.  PWAT values remain
around 1.5-1.75 inches. Guidance has CAPE values around 1000 J/kg,
but fatter CAPE profiles. Shear remains low around 20 kt. Best
coverage appears to be along the I-39 corridor and south of I-80.

Another weaker wave may force showers and storms Tuesday, and I have
less confidence in timing and coverage.  Showers may continue into
Wednesday morning. A cold front stalls to the west of the forecast
area, and guidance differs on if/when showers and storms associated
with the front will impact the region.  Kept a chance of showers and
storms in the forecast Wednesday night, but thinking Thursday will
be mostly dry.

Seasonal to slightly above normal temperatures are expected
throughout the extended.



For the 18Z TAFs...

South to southwest winds continue early this afternoon with mixing
bringing occasional gusts to near 20 kt. Scattered VFR cumulus
will continue through the afternoon with bases rising with time.
Wind speeds will ease slightly this evening with the direction
turning south to southeast before turning more south to southwest
overnight. Showers and thunderstorms are expected to move into
northwest Illinois ahead of a cold front overnight but be on a
decreasing trend. Have added shower mention at RFD late tonight as
some activity does look to hold together. Cannot rule out some
embedded thunderstorms. Less confident in precipitation holding
together toward the Chicago area but did include a mention mid

The cold front will cross the terminals into the late afternoon
turning winds northwesterly. New storm development is possible
along the front from roughly ORD to PNT and points east early to
mid afternoon but it is possible that development holds off a
little longer pushing the threat east of the Illinois terminals.
GYY may have the best threat for thunder of any site by late
afternoon. Expect a mix of clouds and sun and some MVFR stratocu
may build in behind the front during the afternoon.



333 AM CDT

Moderate southerly flow will be in place through at least midday
Sunday on the western edge of ridge of high pressure over the
Appalachians and ahead of an approaching cold front. Winds are
expected to remain in the 15 to 25 kt range through much of the time
frame. The front will make its way down Lake Michigan through the
day Sunday turning winds northerly. High pressure will build in
behind the front and move just north of Lake Michigan on Monday
allowing winds to veer back to southeast by Tuesday. South to
southeast winds will generally prevail through the end of the week.






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